turns out that was a really easy fix

src/lib.rs

@@ -67,7 +67,7 @@
 //! 
 //! Finally, a vector can be `unsize`d to another vector:
 //! 
-//! ```should_panic
+//! ```
 //! # use dyn_vec::prelude::{*, vec};
 //! # use std::fmt::Debug;
 //! let vec: Vec<i32> = vec![1, 2, 3];
@@ -438,10 +438,15 @@ impl<T: ?Sized> Vec<T> {
     }
 
     /// Converts a `Vec<T: Sized>` into a `Vec<U: ?Sized>`, given that `T` can be `CoerceUnsized` into `U`.
-    pub fn unsize<U: ?Sized>(self) -> Vec<U> where for<'a> &'a T: CoerceUnsized<&'a U> {
-        #![allow(unreachable_code)]
-        // FIXME: when fixed, make the 2 relevant tests not should_panic (test::unsize and the line 70 doctest)
-        panic!("Vec::unsize is unsound rn, sorry yall");
+    pub fn unsize<U: ?Sized>(mut self) -> Vec<U> where for<'a> &'a T: CoerceUnsized<&'a U> {
+        if size_of::<Extra<U>>() > size_of::<Extra<T>>() {
+            let elem_size = self.end_ptr.as_ptr() as usize - self.ptr.as_ptr() as usize;
+            let extra_size = self.len * size_of::<Extra<U>>();
+            let needed_size = elem_size + extra_size;
+            if needed_size > self.capacity {
+                unsafe { self.realloc(needed_size); }
+            }
+        }
 
         let new_vec = Vec::<U> {
             ptr: self.ptr,

src/test.rs

@@ -189,7 +189,6 @@ fn with_capacity() {
     assert_ne!(prev_ptr, vec.as_ptr());
 }
 
-#[should_panic = "Vec::unsize is unsound rn, sorry yall"]
 #[test]
 fn unsize() {
     let vec = vec![1, 2, 3];